Light emitting diode having a wavelength shift layer and method of manufacture

ABSTRACT

Embodiments of the present invention provide an LED having a Wavelength Shift Layer (WSL) and method of manufacture. Specifically, under embodiment of the present invention, a WSL layer is applied over an LED chip. The WSL itself typically comprises two layers: an adhesion layer applied over a set (at least one) of LED chips, and a conformal coating over the adhesion layer. The adhesion layer provides improved adhesive effect of the conformal coating to the LED chip(s). The conformal coating is comprised of a particular phosphor ratio that is determined based on a wavelength measurement of the underlying LED chip(s). Specifically, under the present invention, a wavelength of a light output by an LED chip(s) (e.g., blue or ultra-violet (UV)) is measured (e.g., at the wafer level). Typically, the phosphor ratio of is comprised of at least one of the following colors: yellow, green, or red. Regardless, this conformal coating is applied over a glue layer that itself is applied over the LED chip.

CROSS-REFERENCE TO RELATED INVENTION

The present invention is related in some aspects to commonly-owned andco-pending application Ser. No. 12/693,632, filed Jan. 26, 2010, andentitled LIGHT EMITTING DIODE (LED) AND METHOD OF MANUFACTURE, theentire contents of which are herein incorporated by reference. Thepresent invention is also related in some aspects to commonly-owned andco-pending application number (to be provided), filed (to be provided),assigned attorney docket number MOON-0005, and entitled MULTICHIP LIGHTEMITTING DIODE (LED) AND METHOD OF MANUFACTURE, the entire contents ofwhich are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention generally relates to Light Emitting Diodes (LEDs).Specifically, the present invention relates to the manufacture of an LEDhaving a Wavelength Shift Layer (WSL).

BACKGROUND OF THE INVENTION

As LEDs continue to grow in popularity as an efficient technologicalapproach, the need for continued advancement grows as well. Along theselines, obtaining white light output from LED is not only needed, butalso difficult to achieve. Many approaches in the past have attempted tofind new ways to obtain white light. However, many of these approachesperform such processing at the chip level instead of at the wafer level.Such an approach can result in chip waste. Moreover, none of theexisting approaches vary phosphor ratios based on an underlying devicemeasurement (such as a wavelength of a light output). For example, U.S.Pat. No. 6,650,044 forms a pedestal on top of a pad. The pedestal is astud bump that is used for connectivity. This approach is not ideal asit is inefficient, does not provide chip level coating, is much harderto polish, and is easily contaminated. In view of the foregoing, thereexists a need for an LED and associated method of manufacture thataddresses the deficiencies of the related art.

SUMMARY OF THE INVENTION

In general, embodiments of the present invention provide an LED having aWavelength Shift Layer (WSL) and method of manufacture. Specifically,under embodiment of the present invention, a WSL layer is applied overan LED chip. The WSL itself typically comprises two layers: an adhesionlayer applied over a set (at least one) of LED chips, and a conformalcoating over the adhesion layer. The adhesion layer provides improvedadhesive effect of the conformal coating to the LED chip(s). Theconformal coating is comprised of a particular phosphor ratio that isdetermined based on a wavelength measurement of the underlying LEDchip(s). Specifically, under the present invention, a wavelength of alight output by an LED chip(s) (e.g., blue or ultra-violet (UV)) ismeasured (e.g., at the wafer level). Typically, the phosphor ratio iscomprised of at least one of the following colors: yellow, green, orred. Regardless, this conformal coating is applied over a glue layerthat itself is applied over the LED chip.

In one embodiment, an additional protective layer can be applied overthe WSL (i.e., over the conformal coating). Regardless, the light outputof the LED chip is converted to white light using the WSL. In a typicalembodiment, these steps are performed at the wafer level so thatuniformity and consistency in results can be better obtained. Moreover,any quantity of chips (i.e., at least one or a “set”) can besimultaneously coated hereunder. However, it should be understood thatthe same teachings could be applied at the chip level. Moreover, severaldifferent approaches can be implemented for isolating the coating area.Examples include the use of a paraffin wax, a silk screen, or a photoresist.

A first aspect of the present invention provides a method formanufacturing a light emitting diode (LED), comprising: taking ameasurement for a set of LED chips; applying an adhesion layer over theset of LED chips; applying a conformal coating over the conformalcoating having a phosphor ratio that is based on the measurement; andconverting a light output of the set of LED chips to white light usingthe conformal coating.

A second aspect of the present invention provides a method formanufacturing a light emitting diode (LED), comprising measuring awavelength of a light output by a set of LED chips; applying an adhesionlayer over the set of LED chips, the adhesion layer comprising anadhesive selected from a group consisting of: a silicon resin, an epoxyresin, an organic polymer, and a glass resin; applying a conformalcoating to the set of LED chips, the conformal coating having a phosphorratio that is based on the wavelength, and the phosphor ratio comprisedof at least one of the following colors: yellow, green, or red; andconverting the light output of the set of LED chips to white light usingthe conformal coating.

A third aspect of the present invention provides a light emitting diode(LED), comprising: a set of LED chips; an adhesion layer over the set ofset of LED chips; and a conformal coating over the adhesion layer, theconformal coating having a phosphor ratio that is based on a wavelengthof a light output by the underlying set of LED chips, wherein theconformal coating converts the light output of the set of LED chips towhite light using the conformal coating.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of this invention will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings in which:

FIG. 1 depicts a chip wavelength distribution graph in the LED chipwafer;

FIG. 2 depicts a graphic representation of the chromaticity diagram;

FIG. 3 depicts a method of coating fluorescent substance at the existingchip unit PKG level;

FIG. 4 depicts a graphic representation of a chip level conformalcoating (CLCC) mode in the wafer level;

FIG. 5 depicts a method of providing the guide dam in a scribe line andmetal pad part in the wafer level through a paraffin print and coatingthe fluorescent substance plus silicon according to each chip withdispensing in a CLCC implementation;

FIG. 6 depicts a method of making a fluorescent substance plus siliconaccording to each chip with dispensing after making the scribe line andmetal pad part with blocking by using the silk screen or the metal maskin the CLCC implementation;

FIG. 7 depicts a method of making the fluorescent substance plus siliconaccording to each chip with dispensing after making the scribe line andmetal pad part with masking by using the sacrificial photo resist or theorganic compound in the CLCC implementation; and

FIG. 8 depicts an LED chip as coated with a wavelength shift layer andoptional protective layer.

The drawings are not necessarily to scale. The drawings are merelyschematic representations, not intended to portray specific parametersof the invention. The drawings are intended to depict only typicalembodiments of the invention, and therefore should not be considered aslimiting the scope of the invention. In the drawings, like numberingrepresents like elements.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, embodiments of the present invention provide an LEDhaving a Wavelength Shift Layer (WSL) and method of manufacture.Specifically, under embodiment of the present invention, a WSL layer isapplied over an LED chip. The WSL itself typically comprises two layers:an adhesion layer applied over a set (at least one) of LED chips, and aconformal coating over the adhesion layer. The adhesion layer providesimproved adhesive effect of the conformal coating to the led chip(s).The conformal coating is comprised of a particular phosphor ratio thatis determined based on a wavelength measurement of the underlying LEDchip(s). Specifically, under the present invention, a wavelength of alight output by an LED chip(s) (e.g., blue or ultra-violet (UV) ismeasured (e.g., at the wafer level). Typically, the phosphor ratio iscomprised of at least one of the following colors: yellow, green, orred. Regardless, this conformal coating is applied over an adhesionlayer that itself is applied over the LED chip.

In one embodiment, an additional protective layer can be applied overthe WSL (i.e., over the conformal coating). Regardless, the light outputof the LED chip is converted to white light using the WSL. In a typicalembodiment, these steps are performed at the wafer level so thatuniformity and consistency in results can be better obtained. Moreover,any quantity of chips (i.e., at least one or a “set”) can besimultaneously coated hereunder. However, it should be understood thatthe same teachings could be applied at the chip level. Moreover, severaldifferent approaches can be implemented for isolating the coating area.Examples include the use of a paraffin wax, a silk screen, or a photoresist.

A wavelength distribution exists due to an EPI in-line MOCVD (metalorganic chemical vapor deposition) process drift and facility deviationof the LED chip between each chip within a LED chip wafer. FIG. 1depicts the result of measuring the wavelength of blue light for chips10 within an arbitrary LED chip wafer 12. In FIG. 1, chips are unable tohave the same monochromatic light output within the wafer and it has adistribution between chips. In general, in the case of the wave length,a difference is generated between chips that is over 5 nm, and therecognition capability is that of human vision.

FIG. 2 shows a RGB chromaticity diagram. Because the wavelengthdistribution exists similar to FIG. 1 within the wafer between chips, inthe case of applying a fluorescent substance of an identical combinationratio in the wafer level in order to implement white, the white coloroutput coordinate is changed on each chip. That is, the wavelengthdissemination of the white chip is enlarged.

Therefore, only when the combination ratio of the fluorescent substanceapplied according to the wavelength of the chip is appropriate, the sametarget white chip implementation is possible. In the chromaticitydiagram of FIG. 2, the wavelength of chip number 1 is α. This ratio ofyellow, red, and green (Y, R, G) should be applied according tocombination ratio A of the fluorescent substance and silicon to form thewhite target color output coordinate. Moreover, as to chip numbers 2 and3, the wavelengths of β and γ need be applied. To form the same whitetarget color output coordinate as chip number 1, ratios of B and C haveto be applied to chip numbers 2 and 3 (respectively).

If the combination ratio of the coating film (fluorescent substance plussilicon) is identical for α, β, γ in which the chips are differentwavelengths, then the white target color output coordinate of the threechips will be different. There is a difficulty in the BLU (back lightunit), and a lamp in the LED application with the color dispersionoccurrence in the product configuration if the white color outputcoordinate is changed. Before implementing the dispensing process ofcoating the fluorescent substance, the sorting (or ranking) is made ofthe chips for each wavelength of the blue light output from the chips.

Referring now to FIG. 3, the application of WSL 14 is schematicallyshown. It is understood that dispenser 25 can be used to apply some orall of the layers provided hereunder. Specifically, dispenser 25 can beused to apply an adhesion layer over set LED chips 10, a conformalcoating over the adhesion layer, and a protective/passivation layer overthe conformal coating. In any event, as shown, there is lead frame 16including cup 18 and the LED chip 10 is attached in the center of thecup, and the metal pad 20 and lead frame 22 are connected by the wirebond 24. WSL 14 (an adhesion layer and a fluorescent substance plussilicon layer), which fits to the wavelength of the corresponding chip10, and in which it is designed in the LED chip surface in order to formthe white target color output, is coordinated with dispensing. Thisdescribed chip level fluorescent substance coating technology hasseveral problems.

First, because of the thick coating film (over the minimum 300 um) ofthe fluorescent substance plus silicon, the optical mean free path ischanged according to the LED chip surface location and the colordeviation is caused (in other words, binning phenomenon). When itdesigns optically, this color deviation occurrence brings many elements.

Second, the fluorescent substance coating process is not conducted atthe wafer level but is done at the chip level. As a result, the separatepackage material and process cost are additionally generated. Moreover,the chip cost of the inferior chip is generated by the white coloroutput coordinate deviation after the package process. The sortingprocess is classified with chips within wafer accompanied according tothe same wavelength band because the wavelength distribution isquantified in advance according to the chip level coating. In this case,the long process time and equipment investment cost, etc., aregenerated. The separate sorting process is unnecessary if thefluorescent substance application process of each unit chip is conductedat the wafer level. Since it is not the package level, and the whitecolor output coordinate is already obtained from the unit wafer level,the separate package process and material cost are not generated inaddition to the inferior chip performance variation.

As indicated above, under the present invention, the whitelight-emitting diode manufacturing method performs the whitelight-emitting diode manufacturing process in the wafer leveldifferently than the chip level packaging method. In particular, in thedescribed lower-stage wafer level (after the wavelength data of eachchip is measured in advance), in order to control the fluorescentsubstance (yellow, blue, green) combination ratio, in which it isappropriate for each unit chip for the corresponding for eachwavelength, accurately and radiate the white light in the wafer levelaccording to each chip, the appropriately coated film is made withfluorescent substance plus silicon by using the dispensing method. Inthis way, the fluorescent substance conformal coating in which it hasthe thickness fixed, and is thin in the chip surface, is the basis ofthe possible method (chip level conformal coating: CLCC). In the presentinvention, the meaning of the wafer level points to the wafer state thatdiscrete chips do not occur before the dicing process.

In FIG. 4, an additional feature of the present invention isschematically shown. In order to implement the target white color outputcoordinate in which it is identical about all chips 30 in wafer 32, aproper fluorescent substance combination ratio in which it has to becoated in each unit chip by using the result of measuring the wavelengthof all chips 30 is determined. For example, the combination ratio of Ain case of the wavelength α, the combination ratio of B in case of thewavelength β, and the combination ratio of C in case of the wavelength γis applied. Dispensers 34A-C corresponded to the multiple combinationratio and three coating materials are prepared. Each dispenser 34A-C isfilled with the fluorescent substance of the respectively differentcombination ratio A, B, or C.

As seen in FIG. 4, dispenser 34A-C makes the coated film (fluorescentsubstance plus silicon material) in which it corresponds to each unitchip while at the wafer level by the dispensing method. Therefore, it iscomprised of the fluorescent substance conformal coating in the adhesionlayer that was previously applied to the top surface of each LED chip.In this way, the white LED chip is ultimately implemented in the waferlevel through the fluorescent substance coating. Under the presentinvention, three examples in which it independently coats thefluorescent substance plus silicon material in the wafer level accordingto each unit chip 30 are proposed.

The first example is shown in FIG. 5. In this case, the scribe linebetween chips 30 within wafer 32 prints a material that includesparaffin 36 (possibly along with others). This prevents the fluorescentsubstance material from being coated in the scribe line. Next, the metalpad part prints a material including paraffin 36, etc., along the padline. As to this, the fluorescent substance plus silicon material doesnot penetrate into the pad part in the dispensing process. Now, thethickness of the coating film becomes about 100 um in the case of amaterial including paraffin by printing method. A major function of theparaffin material is as a guide dam 38. As to the guide dam 36, thefluorescent substance plus silicon material prevents a coating.Accordingly, in the wafer level, the coating is possible at each chipcorresponding to the fluorescent substance plus silicon material of thedifferent combination ratios for the target white light output. Theconfiguration of this embodiment is as follows:

(1) Measure the wavelength of each diode chip in the wafer level;

(2) Determine the combination ratio of the proper fluorescent substance(yellow, red, green) for making the white target color output coordinatewith the measured blue wavelength general classification and silicon;

(3) Form the guide dam in the scribe line between all chips and theoutside of the pad within the LED chip wafer as shown in FIG. 5 using amaterial including paraffin by being made with the line printing (ordispensing);

(4) Coat the fluorescent substance in which it has the combination ratioappropriate for each chip wavelength within the wafer it implements, thewhite target color output coordinate, and the silicon material for theadhesive force with dispensing; and

(5) Hard cure the fluorescent substance plus silicon material.

The second example is show in FIG. 6. In this embodiment, thefluorescent substance plus silicon material is able to infiltrate indispensing through blocking the scribe line between chips 30 withinwafer 32 and a metal pad part by using the silk screen or the metal mask40, etc. After a silk screen blocking process 40, the fluorescent plussilicon material can be fit to the corresponding wavelength of each unitchip 30 with the dispensing method and the fluorescent substance plussilicon material is coated on each chip 30 in the wafer level. Theconfiguration of this embodiment is as follows:

(1) Measure the wavelength of each comprised diode chip at the waferlevel;

(2) Determine the combination ratio of the proper fluorescent substance(yellow, red, green) for making the white target color output coordinatewith the measured blue wavelength general classification and silicon;

(3) Block the scribe line and pad part in each chip within the wafer asshown in FIG. 6 by using the silk screen and metal mask;

(4) Coat the fluorescent substance in which it has the combination ratioappropriate for each chip wavelength within the wafer it implements, thewhite target color output coordinate, and the silicon material for theadhesive force with dispensing; and

(5) Hard cure the fluorescent substance plus silicon material.

One additional technique is shown in FIG. 7. In this embodiment, thefluorescent substance plus silicon material is able to infiltrate bybeing dispensed between chips 30 within wafer 32 and the metal pad partby using the sacrificial layer photo resist material 42 or organiccompound. Thus, after the masking process, the material fits to thecorresponding wavelength of each unit chip with the dispensing methodand the fluorescent substance plus silicon material is coated on thechip in the wafer level. The configuration of this embodiment is asfollows.

(1) Measure the wavelength of each comprised diode chip in the waferlevel;

(2) Determine the combination ratio of the proper fluorescent substance(yellow, red, green) for making the white target color output coordinatewith the measured blue wavelength general classification and silicon;

(3) Mask the scribe line and pad part in each chip within the wafer asshown in FIG. 7 by using the sacrificial photo resist, etc.;

(4) Coat the fluorescent substance which has the combination ratio whichis appropriate for each chip wavelength within the wafer it implements,the white target color output coordinate, and the silicon material forthe adhesive force with dispensing;

(5) Cure the fluorescent substance plus silicon material in the ovenwith the hard curing; and

(6) Remove the sacrificial photo resist of the scribe line and pad part.

Referring to FIG. 8, a more detailed diagram of a processed LED chipaccording to an embodiment of the present invention is shown. WSL 14 isshown applied over LED chip 30. The application of WSL 14 is typicallyaccomplished as shown and described above in conjunction with FIGS. 1-7(e.g., measuring wavelength of light output for conformal coating,isolating an area of LED chip, applying adhesion layer to LED chip,applying conformal coating over adhesion layer, etc.). In a typicalembodiment, WSL 14 comprises adhesion layer 33 applied over LED chip 30and conformal coating 37 applied over adhesion layer 33. Adhesion layercomprises an adhesive selected from a group consisting of: a siliconresin, an epoxy resin, an organic polymer, and a glass resin. Conformalcoating 37 is typically prepared and applied as described above byhaving a phosphor ratio comprised of at least one of the followingcolors: yellow, green, or red, the particular ratio being based on ameasured wavelength of light output by LED chip 30. FIG. 8 also showsthat an additional protective/passivation layer can be applied (e.g.,via dispenser(s) of FIGS. 3-4) to provide protection for conformalcoating 37. In a typical embodiment, the passivation layer can comprisean adhesive selected from a group consisting of: a silicon resin, anepoxy resin, an organic polymer, and a glass resin.

Thus, as described herein, under the present invention, the whitelight-emitting diode manufacturing method performs the whitelight-emitting diode manufacturing process in the wafer level and isdifferent from the used chip level packaging method. In detail, in thedescription lower-stage wafer level, after the wavelength data of eachchip is measured in advance, in order to control the fluorescentsubstance (yellow, blue, green) combination ratio appropriate for eachunit chip for the corresponding wavelength accurately, and radiate thewhite light in the wafer level according to each chip, the fitfluorescent substance is made with dispensing. In this way, thefluorescent substance conformal coating in which it has the thicknessfixed and thin in the chip surface is the basis of the possible method(chip level conformal coating: CLCC). The invention effects such as thefollowing can be observed.

First, the white light diode processing yield is increased by the colordeviation reduction between the chips because the suitable fluorescentsubstance plus silicon material for the white formation is adjusted inthe wafer level for each unit chip it coats.

Second, the separate sorting process is certainly not needed since thefluorescent substance coating process is made at the discrete chip unitin the wafer level. Consequently, the total process time is shortened.

Third, the process cost and investment cost can be reduced, since it isnot at the PKG level and the white color output coordinate is alreadyobtained from the unit chip at the wafer level. The separate PKG processand material cost are not generated. Therefore, the production equipmentrequirements decrease in investment and white light diode chipmanagement expenses go down since the white light chip is manufacturedat the wafer level.

In an alternative embodiment, a correspondence is easy through the COM(chip on module) in which we use the flip chip mode and the COB (chip onboard), since the white light diode chip is completed for each chip atthe wafer level. As a result, since existing processes, including theseparate die attachment, wiring, the fluorescent substance coating,etc., can be skipped, the process cost, the material cost, etc., can bereduced.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed and, obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to aperson skilled in the art are intended to be included within the scopeof the invention as defined by the accompanying claims.

1. A method for manufacturing a light emitting diode (LED), comprising:taking a measurement for a set of LED chips; applying an adhesion layerover the set of LED chips; isolating an area of the set of LED chipsusing at least one of: paraffin wax, a silk screen, and a photo resist;applying a conformal coating to the isolated area over the adhesionlayer, the conformal coating having a phosphor ratio that is based onthe measurement; and converting a light output of the set of LED chipsto white light using the conformal coating.
 2. The method of claim 1,the adhesion layer comprising an adhesive selected from a groupconsisting of: a silicon resin, an epoxy resin, an organic polymer, anda glass resin.
 3. The method of claim 1, the measurement comprising ameasurement of a wavelength of the light output by the set of LED chips.4. The method of claim 1, further comprising applying a passivationlayer over the conformal coating.
 5. The method of claim 1, the set ofLED chips comprising a blue set of LED chips.
 6. The method of claim 1,the set of LED chips comprising an ultra-violet set of LED chips. 7.(canceled)
 8. (canceled)
 9. (canceled)
 10. (canceled)
 11. The method ofclaim 1, the conformal coating having a phosphor ratio comprised atleast one of the following colors: yellow, green, or red.
 12. The methodof claim 1, the set of LED chips being disposed on a wafer with aplurality of other sets of LED chips, the conformal coating beingapplied to the wafer.
 13. A method for manufacturing a light emittingdiode (LED), comprising: measuring a wavelength of a light output by aset of LED chips; applying an adhesion layer over the set of LED chips,the adhesion layer comprising an adhesive selected from a groupconsisting of: a silicon resin, an epoxy resin, an organic polymer, anda glass resin; isolating an area of the set of LED chips using at leastone of: paraffin wax, a silk screen, and a photo resist; applying aconformal coating to the isolated area of the set of LED chips, theconformal coating having a phosphor ratio that is based on thewavelength, and the phosphor ratio comprised of at least one of thefollowing colors: yellow, green, or red; and converting the light outputof the set of LED chips to white light using the conformal coating. 14.The method of claim 13, further applying a passivation layer over theconformal coating.
 15. (canceled)
 16. (canceled)
 17. (canceled) 18.(canceled)
 19. (canceled)
 20. (canceled)